1. Field of the Invention
The invention relates generally to radio communication and more specifically to dual-frequency microwave antenna systems that operate with signals received from orbiting navigation satellites.
2. Description of the Prior Art
The global positioning system (GPS) supported by the United States government enables satellite navigation for military and civilian users alike. Two codes, a coarse-grained acquisition code (C/A-code) and a fine-grained precision code (P-code) are transmitted on two L-band microwave frequencies, e.g., L1 on 1575.42 MHz and L2 on 1227.6 MHz, and each provide ranging and GPS-system time information. The C/A-code is available to civilian users and the P-code is available primarily to authorized users. During certain periods, the P-code is encrypted and such encrypted P-code is referred to as Y-code. The C/A-code is a digital sequence that repeats each millisecond and is unique to one of two dozen satellites. The P-code is a digital sequence that has a period of 269 days, with one week long segments of it transmitted intact. Which segment is being sent in any one week is classified. A single week-long segment is 10.23.times.10.sup.6 .times.604800 bits long, which comes from the P-code transmission rate of 10.23 MHz times the exact number of seconds in seven whole days. So a code phase uncertainty of even .+-.1 second can call for a search through 20,460,000 chips.
The L1 and L2 carrier frequencies are sufficiently separated in the radio spectrum as to require separate antennas for their reception. Microwave patch antennas to receive L-band radio transmissions are conventional and comprise a rectangular conductive foil on a flat ceramic substrate. With a center feed, such antennas behave as two dipole antennas set at right angles. With a square conductive foil, the two dipole antennas will have the same frequency response. With a rectangular conductive foil, the two dipole antennas will have different frequency responses, depending on the respective lengths of the sides of the rectangle formed.
Prior art dual-frequency GPS antennas include a stacked array, where two microwave patch antennas, one for L1 and one for L2, are stacked one in front of the another, with their respective planes in parallel. A single down-feed is connected to a conventional navigation satellite receiver.
Authorized GPS users need to be able to switch between receiving the L1 and L2 carrier frequencies. Acquisition of the P-code on the L2 carrier often requires first bootstrapping through the C/A-code on the L1 carrier frequency. Sometimes jamming or spoofing will be present on one of the carrier frequencies but not the other. Some receivers need to peak at the other carrier frequency briefly to measure the automatic gain control (AGC) voltage of the intermediate amplifier.